A great many IMDs for cardiac monitoring and/or therapy comprising sensors located in a blood vessel or heart chamber coupled with an implantable monitor or therapy delivery device have been proposed or implemented. For example, such cardiac systems include implantable heart monitors and therapy delivery devices including pacemakers, cardioverter/defibrillators, heart pumps cardiomyostimulators, ischemia treatment devices, and drug delivery devices. Most of these cardiac systems include electrodes for sensing and sense amplifiers for recording and/or deriving sense event signals from the intracardiac or remote electrogram (EGM). In current cardiac IMDs providing a therapy, the sense event signals are utilized to control the delivery of the therapy in accordance with an operating algorithm and at least selected EGM signal segments and sense event histogram data or the like are stored in internal RAM for telemetry out to an external programmer at a later time. In the MEDTRONIC.RTM. Reveal.TM. implantable loop recorder, a 42 minute segment of EGM is recorded when the patient feels the effects of an arrhythmic episode and activates the recording function by applying a magnet over the site of implantation, but this device provides no therapy.
Efforts have also been underway for many years to develop implantable physiologic signal transducers and sensors for temporary or chronic use in a body organ or vessel usable with such IMDs for monitoring a physiologic condition other than or in addition to the EGM to derive and store data and/or to control a therapy delivered by the IMD. A comprehensive listing of implantable therapy delivery devices are disclosed in conjunction with implantable sensors for sensing a wide variety of cardiac physiologic signals in U.S. Pat. No. 5,330,505, incorporated herein in its entirety by this reference.
Blood pressure and temperature signal values respond to changes in cardiac output that may be caused by a cardiac failure, e.g., fibrillation or high rate tachycardia, or that may reflect a change in the body's need for oxygenated blood. In the former case, monitoring of a substantial drop in blood pressure in a heart chamber, particularly the right ventricle, alone or in conjunction with an accelerated or chaotic EGM, was proposed more than thirty years ago as an indicia of fibrillation or tachycardia sufficient to trigger automatic delivery of defibrillation or cardioversion shock. More recently, it has been proposed to monitor the changes in blood pressure (dP/dt) that accompany normal heart contraction and relaxation and blood pressure changes that occur during high rate tachycardia and fibrillation or flutter.
A number of cardiac pacing systems and algorithms for processing the monitored mean and dP/dt blood pressure have been proposed and, in some instances employed clinically, for treating bradycardia. Such systems and algorithms are designed to sense and respond to mean or dP/dt changes in blood pressure to change the cardiac pacing rate in a rate range between an upper and a lower pacing rate limit in order to control cardiac output. Similarly, a number of cardiac pacing systems have been proposed, e.g., the system disclosed in U.S. Pat. No. 4,436,092, incorporated herein by reference, and, in some instances employed clinically, that sense and respond to changes in blood temperature to change the cardiac pacing rate in a rate range between an upper and a lower pacing rate limit in order to control cardiac output.
With respect to cardiac monitoring, it has been proposed to sense and record such additional physiologic signals including blood pressure in or adjoining blood vessels and heart chambers during the cardiac cycle, blood temperature, blood pH, and a variety of blood gases. Implantable heart monitors and blood pressure and temperature sensors that derive absolute blood pressure signals and temperature signals are disclosed in commonly assigned U.S. Pat. Nos. 5,368,040, 5,535,752 and 5,564,434, and in U.S. Pat. No. 4,791,931, all incorporated by reference herein.
The leads and circuitry disclosed in the above-incorporated, commonly assigned, '752 and '434 patents can be employed to record the EGM and absolute blood pressure values for certain intervals. The recorded data is periodically telemetered out to a programmer operated by the physician in an uplink telemetry transmission during a telemetry session initiated by a downlink telemetry transmission and receipt of an interrogation command.
Certain of the measured physiologic signals derived from the heart or blood in the circulatory system are affected by ambient conditions that cannot be separately measured by the above-described IMDs and physiologic sensors. Specifically, blood pressure and temperature signal values derived by a wholly implantable system are affected by atmospheric pressure acting on the patient and ambient temperature or by a fever afflicting the patient, respectively. In addition, certain implantable blood pressure sensors, e.g., those disclosed in the above-incorporated, commonly assigned '434 and '752 patents, are also affected by blood temperature changes.
Changes in ambient conditions other than weather changes can also influence the measurement of absolute blood pressure changes, including both mean or average blood pressure and dP/dt pressure changes, by implantable pressure sensors. For example, when a patient in which such an implantable blood pressure sensing medical device is implanted changes elevation by ascending or descending in an elevator in a tall building or in an airplane, the change in barometric pressure changes the absolute blood pressure sensed in the body by an amount that can mask changes that are sought to be measured. In the context of an implantable rate responsive pacemaker operating under a rate control algorithm, the pressure change caused by the elevation change itself may exceed the blood pressure change that reflects a change in exercise level of the patient and be mis-interpreted as meriting a change in pacing rate to the upper or lower pacing rate limit, which can, at least, be uncomfortable to the patient. The barometric pressure effect can similarly have a negative effect on operating and detection functions of other IMDs reliant on accurately sensing cardiac blood pressure changes that truly reflect a cardiac function or requirement for cardiac output.
The effect of barometric pressure on cardiac blood pressure measurements has been noted. In commonly assigned U.S. Pat. No. 4,407,296, a micro-machined pressure sensor is disposed at the distal end of a lead in an oil filled chamber on one side of a pressure sensor element that is closed by a flexible membrane that is perpendicular to the lead body axis. The membrane is disposed behind a protective grill at the distal tip of the lead within which blood fluids can contact the exposed side of the membrane . Blood pressure changes deflect the membrane, and the deflection is transmitted through the oil to the micro-machined pressure sensor element which is deflected to produce a pressure signal value change proportional to the blood pressure change acting on the membrane. The blood pressure change reflects both the blood pumping action of the heart and the ambient atmospheric pressure acting on the patient's body. In a first embodiment, the affect of atmospheric pressure is attempted to be offset by providing a chamber behind the sensor element that is sealed at a known average atmospheric pressure.
In practice, this approach has proven to be inadequate because the known pressure has accounted adequately for changes in barometric pressure and renders the blood pressure measurements ambiguous.
In a second embodiment, the chamber behind the sensor element is filled with oil and extends proximally through a lumen of the lead body to a further membrane or diaphragm near the proximal end of the lead body that is to be positioned in the subcutaneous cavity under the patient's skin where the implantable monitor or pulse generator is implanted. In this case, the membrane on the lead body is difficult to manufacture, fragile and can become obstructed in chronic implantation. Moreover, the oil filled lumen can be generally either vertical or horizontal in all or in part depending on a number of factors, including the implantation path of the lead body between the subcutaneous cavity and the implantation site of the pressure sensor in the patient's heart chamber and whether the patient is upright or supine. The weight of the oil in the oil filled lumen depends on the orientation of the lumen with respect to the force of gravity, and the variable weight itself biases the pressure sensor element in a variable manner. Therefore, the reference pressure varies unpredictably and may not represent barometric pressure.
In recognition of these problems with absolute pressure sensors employed to measure blood pressure in a heart chamber or blood vessel, it is suggested in the above-incorporated, commonly assigned, '752 and '434 patents that the patient be provided with a belt worn, external pressure recorder that records and time stamps recordings of barometric pressure that can be retrieved and used as reference pressure data for comparison with the internally recorded absolute blood pressure data. Such an externally worn, barometric pressure recorder is intended to be used with implantable hemodynamic recorders and monitoring IMD's. The reference pressure recordings that are periodically stored in the memory of the external device are read out at the time that the absolute pressure data stored in the implantable monitor is telemetered out. The reference values are subtracted from the absolute values to derive the relative pressure values.
Despite the considerable effort that has been expended in designing such IMDs and associated sensors for sensing such physiologic signals, a need exists for a system and method for accounting for ambient conditions surrounding the patient that affect the sensed and measured physiologic signal values, particularly in the case of pressure, e.g., cardiac blood pressure, other fluid pressures in the body, and optionally temperature.